1. Field of the Invention
The present invention relates to a method and apparatus for safety control for numerically controlled machine tools and, more particularly, pertains to a method and apparatus of sequential control for safety for controlling sequence operations of the numerically controlled machine tools.
2. Description of the Prior Art
In numerically controlled machine tools, a variety of operations such as starting, moving and stopping of elements are sequentially performed under sequential control. Such a numerically controlled machine tool is provided with an NC means which is equipped with external operation functions such as T function (tool function) and M function (miscellaneous function) to control movable elements of the machine tool. The arrangement is such that an operation completion signal is sent from the machine tool to the NC means after each completion of operations to control sequence operations according to the external functions.
Heretofore, it has been disadvantageous that the numerically controlled machine tool of such arrangement will apparently remain stopped if an operation completion signal is not sent to the NC means when something is wrong with the machine tool. The trouble is that the machine tool which has apparently remained stopped will suddenly start to move contrary to the expectation of the operator of the machine tool if the cause for which the operation completion signal has not been sent to the NC means is removed for some reason. This is of course very dangerous to the operator who will be checking most of the time the machine tool to find out the cause for which it has been stopped.